1. Field of the Invention
The present invention relates to a storage system and a storage system control method.
2. Description of the Related Art
In a storage system, a large-capacity storage device can be used to provide storage service to many host computers. An example of a large-volume storage device, there is known a disk array device in which a plurality of disk drives arranged in an array pattern are operated based on a RAID (Redundant Array of Independent Inexpensive Disks).
The storage system provides a snapshot creation function, in order to achieve a high availability and the like. A “snapshot” is a still image of data at the time when the snapshot is created. Snapshots can be divided into volume snapshots and differential snapshots.
A “volume snapshot” is a technique in which the same data is held in both a primary volume and a secondary volume, and the data is duplicated (“mirroring”) to obtain the snapshot (Japanese Patent Application Laid-open No. 2002-373093, Official Gazette). When the snapshot is to be obtained, a link between the primary volume and the secondary volume is terminated (split), and the secondary volume which has been separated from the primary volume is used as the snapshot. The secondary volume is a copy of the primary volume at the time when the snapshot was created. By using the secondary volume to make a backup, the primary volume can be used to provide the storage service even while backup processing is being performed. Once the backup is complete, the data that was updated in the primary volume while the volumes were split is then reflected in the secondary volume, and then the duplication starts again.
The “differential snapshot” refers to a technique in which information about altered locations on the primary volume are managed to achieve the snapshot of a given point in time. For example, when updating the data stored in the primary volume, the pre-update data (old data) is evacuated into the differential volume, and new data is written into the primary volume. Then, a table or the like is used to manage management information relating to the location where the data was updated, the destination to which the data was evacuated, and the like. Accordingly, data constructions at specific points in time can be managed over multiple generations (multiple points in time).
The volume snapshot copies the entire volume at a certain point in time. By using the volume snapshot created at a time desired by the user, the data group from that point in time can be used immediately. Furthermore, since the volume snapshot copies the entire volume, the volume snapshot is very reliable when disk damage and the like occur. However, the volume snapshot requires the same amount of volume as the volume where the data was copied from. Therefore, the greater the number of volumes and number of generations in the snapshot, the more volume becomes necessary. Since a large amount of volume becomes necessary, the snapshot can be taken across relatively few generations.
On the other hand, the differential snapshot separately extracts and manages only the information pertaining to the location that was updated, which can reduce the volume capacity that is needed to create the snapshot, as compared to the volume snapshot. Furthermore, since the differential snapshot extracts and manages the information pertaining to the location where the alteration occurred, the snapshot can be obtained faster than when using the volume snapshot. However, with the differential snapshot, for example, when double disk damage occurs and the differential data is lost, the snapshot is also lost at that point in time. Therefore, the differential snapshot is less reliable when disk damage occurs, as compared to the volume snapshot. Here, “double disk damage” refers to the simultaneous occurrence of multiple damage in the group of disks constituting the RAID group. Furthermore, since the capacity of the differential volume is set relatively small, there is a chance that the differential snapshot will be created in excess of the capacity of the differential volume. When this occurs, a previous differential snapshot is overwritten by the new differential snapshot, whereby the previously obtained differential snapshot is lost.